Control of trains and their movements along defined rails has been a priority since the inception of railroads. Safety concerns are many including concerns about collisions at train crossings, overtaking and colliding with stopped or slower moving trains, head on collisions when trains are traveling on the same rail in opposite directions, improper switch orientation causing trains to enter onto and travel on wrong tracks creating risk of running to the end of the track or colliding with other trains, apparatus or vehicles on the improper track, colliding with road traffic such as cars and trucks at railroad-road crossings and detection of wheel derailments and hot bearings before more serious problems result. In addition to safety concerns, control of trains is highly desirable for efficient and concentrated use of rail lines, e.g. train and car identification and location and speed of movement.
There has therefore been a continuing need for automatically detecting the presence of trains, for detecting train lengths, for determining train speeds, for detecting direction of movement and for detecting bearing problems (i.e. hot boxes) and derailments. Some earlier devices for detecting the presence of trains included pressure switches that operated upon movement of a track section due to train weight and electrical contact switches that operated conduction through train wheels at electrically insulated rail sections. These systems, while better than no automatic detection systems, had serious disadvantages. In particular, pressure switches required expensive and cumbersome apparatus of a size capable of withstanding tremendous train weight and were subject to serious maintenance problems. Further such systems could not detect train direction without using multiple costly pressure switches spaced at significant distances. Such pressure switches could not be used to determine train length, train speed or derailment conditions. Electric contact switches had the disadvantages of pressure switches and in addition required insulated rails subject to insulation breakdown and the possibility of grounding out due to conductive articles or substances, e.g. water or even snow, in contact with the rail.
Another type of detector that has been tried is the photoelectric detector. Such detectors do not work well in an environment where dirt or snow can easily block a photodetector and photodetectors are usually sensitive to shock and vibration. A further type of detector relies upon reflected radio frequency waves and resulting phase shift to determine presence and direction of a train wheel e.g., as described in U.S. Pat. No. 6,043,774. Such detectors have an advantage in that they can be small and use low power but have a serious disadvantage in that they will detect essentially anything whether magnetic or not or massive or not thus resulting in undesirable false positives. Further, such detectors are subject to radio wave interference from extraneous sources such as radio transmitters used by railroad personnel.
In the prior art, train wheel detectors using simple self controlling flux generators having inductance-capacitance tank circuits as sensors were too unreliable for use because of tendency of flux levels and detection levels to drift thus resulting in no reliable standard to use as a basis for comparison when a train wheel entered the flux zone. Such drift resulted from a number of factors including temperature changes that altered component characteristics, presence of iron shavings or powder on the sensors, minor shifting of the sensor relative to the rail, and alteration of characteristics due to component aging. Such flux modification detectors further did not naturally contain fail safe mechanisms indicating when they were operating improperly.
Nevertheless, attempts have been made to detect the presence of train wheels on a track by their affect upon a local electromagnetic field and a number of patents have been granted in this area. Such detectors are either unreliable, for reasons previously stated or are costly and complex due to attempts to overcome the disadvantages previously described. A number of such patents require both a field generator, such as a coil or permanent magnet and at least one detection coil that detects a change in flux density when a wheel flange approaches the coils. The use of both a field generator and a detection coil, or other multiple coil systems not only increases cost and complexity, the detectors are not as sensitive as desired. Examples of patents using multiple coils and or permanent magnets include U.S. Pat. Nos. Re 30,012; 3,697,745; 4,283,031; 4,524,932; 5,333,820; 5,628,479 and European Patent Application 0 002 609. Other systems, e.g. have employed the use of phase shift in an attempt to detect the presence of a train wheel. Such systems are subject to interference and are complex, e.g. as described in U.S. Pat. Nos. 5,395,078 and 3,721,821. A number of systems do not provide for compensation due to environmental factors and component aging, e.g. as described in U.S. Pat. No. 3,941,338, and still others use complex and unreliable circuitry where a microprocessor or other device is used to provide frequency generation that is then fed into a tank circuit, rather than relying upon a tank circuits own natural frequency. Examples of such patents include U.S. Pat. No. 6,371,417 and French patent application 80 25496.
Up to now, no known system has had the desired combination of properties of simplicity, reliability, including compensation, direction detection, and fail safe detection afforded by the apparatus and method of the present invention.